Conventionally, as a fixing apparatus used in an electrophotographic image forming apparatus such as a copying machine, a printer, and the like, the following type of fixing apparatus has generally been used. Specifically, this conventionally-used fixing apparatus (heat roller fixing apparatus) includes a fixing roller and a pressing roller which are pressed against each other, wherein heating means constituted of a halogen lamp (halogen heater) disposed in the fixing roller or halogen lamps (halogen heaters) disposed in both of the rollers heat(s) the pair of rollers at a predetermined temperature (fixing temperature), and a recording paper on which an unfixed toner image is formed is fed to a pressing section (fixing nip section) of the pair of rollers, and the recording paper is allowed to pass through the pressing section so as to fix the toner image by heat and pressure.
In a fixing apparatus provided in a color image forming apparatus, it is general to use an elastic roller having an elastic layer which is made of silicone rubber or the like and which is provided on a fixing roller surface layer.
The elastic roller is used as the fixing roller, so that the fixing roller surface is elastically deformed corresponding to an uneven surface of the unfixed toner image and is in contact with the toner image so as to cover the toner image. This allows the color unfixed toner image whose toner amount is larger than that of monochrome to be favorably heated and fixed. Further, due to strain release of the elastic layer which occurs in the fixing nip section, it is possible to improve a releasing property with respect to color toner which is more likely to be offset than monochrome toner. Further, a nip shape of the fixing nip section has a concave upward (on the side of the fixing roller) (that is, the nip shape is a so-called inverse nip shape), so that it is possible to more favorably strip paper. As a result, it is possible to strip paper without using any stripping means such as a stripping protrusion (self stripping), so that it is possible to prevent insufficient image formation which is caused by the stripping means.
Note that, an image forming apparatus using monochrome toner is also arranged so that the elastic layer is provided only on the pressing roller without being provided on the fixing roller so as to secure the fixing nip.
Incidentally, the fixing roller or the pressing roller which includes the elastic layer raises the following problem. In the fixing roller having the elastic layer, the elastic layer cannot sufficiently conducts heat. Thus, in case where the heating means is provided in the fixing roller, heat is less efficiently conducted, so that it takes longer time to warm up. In case of allowing sheets to sequentially pass at high speed, the temperature of the fixing roller drops.
As a method for solving these problems, a technique in which external heating means is brought into contact with the fixing roller surface so that the fixing roller is heated from the outside (external heat fixing process) is known.
For example, Patent Document 1 (Japanese Unexamined Patent Publication No. 198659/2004 (Tokukai 2004-198659) (Publication date: Jul. 15, 2004)) proposes a technique (an external belt heat fixing process) using an external heating belt (endless belt), suspended by belt suspending rollers, as external heating means. In this technique, the external heating belt is used as the external heating means, so that a contact area between the external heating means and the fixing roller increases, thereby promoting supply of heat from the external heating means to the fixing roller.
Moreover, Patent Document 2 (Japanese Unexamined Patent Publication No. 345084/2003 (Tokukai 2003-345084) (Publication date: Dec. 3, 2003)) discloses a technique of dispersing graphite in a polyimide belt having a thickness of 5 μm to 500 μm, as a fixing belt or a transfer belt.
Moreover, Patent Document 3 (Japanese Unexamined Patent Publication No. 215238/2005 (Tokukai 2005-215238 (Publication date: Aug. 11, 2005)) discloses an endless belt formed by applying a release layer in such a state that the ratio of imidization of a heat-resistant resin layer is 20% to 70%, and burning it.
However, in the technique of Patent Document 1, the external heating belt is heated by heating the belt suspending rollers and heat is supplied to the fixing member via the external heating belt, so that it is necessary to set temperature of each of the belt suspending rollers and the external heating belt to be higher than the surface temperature of the fixing roller. Thus, also after stopping the heating of the belt suspending rollers, each belt suspending roller has higher temperature than the surface temperature of the fixing roller for a while. As a result, when the rotation of the fixing roller is stopped at the time of fixing-standby after finishing the fixing, a contact portion between the fixing roller and the external heating belt (particularly, a portion being in contact with the belt suspending roller via the external heating belt) is locally heated, so that the surface of the fixing roller has uneven temperature, which results in uneven gloss of an image.
Moreover, in the technique of Patent Document 1, the regulation of position of the belt in an axis direction is carried out by adjusting the parallelism of the roller. To regulate the position of the belt by actively adjusting the parallelism of the roller, detection and feedback control are required. This increases the complexity of the apparatus.
Patent Document 1 describes that the polyimide belt is used as the external heating belt, but does not adequately describe the material or strength of the polyimide belt, or an arrangement for securing an adequate life of the belt along with a simple method for regulating the position of the belt.
Moreover, Patent Document 2 discloses an addition of an additive to give electrical conductivity and/or heat conductivity to a polyimide resin belt. However, this belt is used as a fixing belt or a transfer belt. Therefore, Patent Document 2 does not disclose an arrangement for securing an adequate life of a belt which is used as the external heating belt for the fixing member and has a simple arrangement.